No, you just can't get past the brush settings and you obsess over them (more like pixel settings, because really, that's what digital drawing really is).>>3514042
Stop pressing down so hard on the lead. 0.3 and 0.5mm leads are not the kind of lead you press down on to get darker lines unless you have a 2B or darker lead in your mechanical pencil. Standard HB lead can only give so much before it breaks.
Use a 0.9mm mechanical pencil to accomplish darker lines or a good old standard HB wood pencil.